What is the startling revelation at the end of The Wife?

Meg Wolitzer's novel The Wife is a memoir that tells about the life of Joan Castleman. Told through the perspective of Joan, the wife of famous American author Joseph (Joe) Castleman, the book relies on Joan's flashbacks to explain the decisions in their lives leading them to his point.
After fifty years of marriage and three children, Joan has realized that she must leave him. As the book opens, the couple is on a plane to Helsinki, Finland, where Joe will be awarded the Helsinki Prize for Literature. During her reflections, Joan realizes that her life has been an unhappy one; she has continued to put his needs and career ahead of her own, and she is tired of his philandering ways. She describes him as

one of those men who own the world . . . who has no idea how to take care of himself or anyone else, and who derives much of his style from the Dylan Thomas Handbook of Personal Hygiene and Etiquette.

The revelation is that Joe isn't really the author everyone thinks he is; Joan is actually the one who has written all of his books. Once they land in Helsinki, Joan tells Joe that she no longer wishes to be married to him and threatens to tell everyone the truth about their books. Despite his limited plot suggestions, she is the true author. The fear of being found out sends Joe into a panic, and he has a heart attack.
The book ends with Joan returning home to the US feeling a mixture of both relief and sadness. She decides not to tell the truth about Joe's books and wonders what it is that creates a successful life. She decides, even at her advanced age, to pursue her own literary career.